Seasonal Affective Disorder (SAD) is a condition which affects people on an annual basis and is characterized by retarded depression, overeating, oversleeping and carbohydrate craving in the winter and euthymia or hypomania in the spring and summer. It resembles the annual rhythms of reproduction, which are widespread among other animals, and like these rhythms can be modified by extending the winter photoperiod. We have shown that three hours of bright environmental light before dawn and after dusk frequently reverses the symptoms of SAD. In other species the pineal hormone, melatonin, has been shown to mediate the effects of photoperiod. In certain studies the duration of the melatonin pulse, which is inversely related to the duration of the photoperiod, has been shown to be the critical aspect of melatonin secretion, which influences photoperiodic responses. In hamsters and sheep, for example, melatonin administered orally in the evening during the long summer days has been shown to elicit short-day, winter responses. We have explored whether melatonin, administered orally in the evening and morning hours, will reverse the effects of extending the photoperiod with bright light. In eight SAD patients, melatonin and placebo were given for one week each after patients had been successfully treated with bright light. Melatonin did not fully reinduce the typical winter depression but did induce some of its classical symptoms such as overeating, fatigue and oversleeping. Further studies will explore whether patients can be helped by blocking melatonin secretion.